4. LINE EMISSION FROM THE INTERSTELLAR MEDIUM

Until recently, the situation concerning detection of line emission from
hot gas has been murky. The tentative detection by
Feldman et al. (1981)
was not confirmed by
Murthy et al. (1989).
Martin and Bowyer (1990)
present a spectrum for one of their
UVX targets that contains a quite impressive and convincing CIV 1549
Å line, and
a much less impressive O III] 1663 Å line. For their other targets,
for some of which
similar line emission was claimed, Martin and Bowyer present only tiny
portions of
their spectra, at the location of the claimed lines. The full spectra of
all targets should be published.

The recent development on this topic is detection of very strong line
emission in the Eridanus region by
Murthy, Im, Henry, and
Holberg (1993)
using the Voyager
spacecraft. Figure 2 shows the spectrum of their
Target B, which shows strong emission
lines of O VI 1032 / 1038 Å and C III 977 Å radiation. The
lines are extremely strong, and
predicted associated lines (see figure) should be detectable even with
IUE. The region
involved is one where there is a very strong soft X-ray enhancement
(Burrows et al. 1993)
and based on our other Voyager spectra is not typical of the general
interstellar
medium. What this suggests is that when a high-sensitivity sky survey
(e.g.,
Kimble et al. 1990)
is finally carried out, what will be revealed is a highly patchy structured
hot interstellar medium.

Of course the highly structured character of the spectrum of
Figure 2 shows that
there is no possibility that the background at high latitudes is line
emission. (To verify
this, the reader should consult the individual spectra from the relevant
references in
Figure 1, rather than rely on
Figure 1 itself.)

Figure 2. Spectrum of a region in Eridanus
observed by
Murthy, Im, Henry,
and Holberg (1993).
Strong solar system Lyman
(1216 Å) has been
subtracted. Emission
lines of C III (977 Å) and O VI (1032/1038 Å) are seen. The
solid line shows the emission that is expected
(Hartigan et al. 1987)
from a shock with a velocity of 180 km
s-1, including two photon emission, plus appropriate
dust-scattered light. The sensitivity
of Voyager above 1200 Å is too low to allow detection of
additional predicted lines
of N V, C II, Si IV, O IV, and C IV, but those lines should be
accessible to IUE and to the Hopkins Ultraviolet telescope.